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fungi are eukaryotic organisms that possess a ____ cell wall and produce ____ structures and _____

fungi are eukaryotic organisms that possess a chitinous cell wall and produce filamentous structures and spores


describe the structure and composition of the fungal cell wall

  • chitin, B (1,3) and B(1,6) glucans, mannose modified proteins and glycosylphosphatidylinositol (GPI)-anchored proteins
  • the cell membrane has ergosterol instead of cholesterol


describe fungal morphology

  • fungi grow as unicellular (yeast) or multicellular organisms (mold)
  • multicellular fungi connect together in long strands known as hyphae; hyphae grow at their tips and form branches--tangled masses of hyphae are mycelia
    • hyphae may have walls between neighboring cells, making them septate or aseptate
    • aseptate hyphae are referred to as coenocytic, in that a single plasma membrane surrounds many nuclei creating a multinucleate cell
  • fungal nuclei are haploid except during the generation of a zygote
    • a fungal cell with 2 haploid nuclei is known as dikaryon


describe the 4 major types of fungal diseases

  • hypersensitivity: allergic reaction to molds and spores e.g. farmer's lung, tea picker's lung, bark stripper's disease
  • mycotoxicoses: poisoning of man/animal by feeds/products contaminated by toxin producing fungi that colonize crops such as grains, corn, peanuts; lethal to poultry and livestock e.g. aflatoxin, ergot alkaloids
  • mycetismus: ingestion of pre-formed toxins (mushroom poisoning)
  • infections: mycoses


describe fungal classification based off morphology


give examples of the different fungal classifications

  • ascomycetes form sexual spores in an ascus, an enclosed sac
    • largest fungal phylum and includes Candida albicans, Pneumocystis jirovecii and most of the human fungal pathogens
  • basidomycetes form sexual spores on the surface, known as a basidium
    • Cryptococcus neoformans
  • Zygomycetes is the most primitive fungal phylum, forming sexual spres in an enclosed sac known as zygosporangium
    • notable pathogens are Mucor spp., Rhizopus spp. and Absidia spp. 


explain the concept of dimorphism

dimorphism enables switching from one form to another

  • primary trigger = environmental conditions
    • temperature
    • pH
    • iron levels


describe the 3 mechanisms of fungal reproduction

  • 1) production of spores: dispersal units
    • can germinate > new hyphae
    • formation can be by:
      • asexual process (mitosis only)
        • sporangiospores
        • conidiospores
      • sexual process 
        • ascospores
        • basidospores
  • 2) budding
  • 3) hyphal fragmentation
    • germinate from new hyphae


which term best describes fungal growth requirements?

  • saprophytic: obtain nutrients and energy from dead or decaying organic material
    • occassionally human tissue -> damage


describe the superficial mycoses

colonization of the outer layers of skin, hair and nails, and rarely invades deeper tissues

  • pityriasis (Tinea) versicolor caused by the dimorphic Malassexia furfur, which infects skin and alters color
    • M. furfur flouresces under UV light
  • Tinea Nigra, caused by Hortaea werneckii, causes skin to darken
  • black piedra is a superficial infection of the hair shaft caused by Piedra hortae
  • onychomycosis is fungal infection of fingernails and toenails caused by Trichophyon rubrum and T. mentagrophytes


describe subcutaneous mycoses

  • sporotrichosis is a subacute or chronic granulomatous infection, often following lymphatics, caused by the soil fungus Sporothrix schenckii
  • a mycetoma (or eumycetoma) is a granulomatous inflammation that may extend beneath the subcutaneous region to bone
    • pigmented nodules may drain through sinuses and produce colored grains which are helpful in identification
  • chromoblastomycosis is an infection that forms warty pigmented lesions which grow outward from site of introduction


name the 5 common opportunistic mycotic infections

  • Candidiasis 
  • Aspergillosis
  • Cryptococcosis
  • Zygomycosis/mucormycosis
  • Pneumocystis


describe Candidiasis (common opportunistic mycotic infxn)

  • candidiasis:
    • Candida albicans causes superficial skin infection, oral cavity, genitalia, large intestine
    • forms off-white, pasty colony with a yeasty odor
    • causative agent of thrush, vulvovaginal yeast infection and cutaneous candidiasis


describe Aspergillosis (common opportunistic mycotic infection)

  • aspergillosis:
    • very common airborne soil fungus, usually infecting lungs
    • serious opportunistic threat to AIDS, leukemia, and transplant patients
    • invasive aspergillosis can involve many organs


describe Cryptococcosis (common opportunistic mycotic infxn)

  • cryptococcosis:
    • Cryptococcus neoformans inhabits soil around pigeon roosts, usually infecting lungs and other organs, notably, brain
    • common infxn of AIDS, cancer and diabetes patients


describe Zygomycosis/mucormycosis (common opportunistic mycotic infxn)

  • zygomycosis/mucormycosis
    • saprobic fungi found in soil, water, organic debris and food e.g. Rhizopus, Absidia and Mucor
    • usually harmless but can invade nose, eyes, heart and brain of people with diabetes and malnutrition with severe consequences


describe Pneumocystis (common opportunistic mycotic infxn)

  • Pneumocystis:
    • Pneumocystis jiroveci, a small unicellular fungus that causes penumocystis pneumonia (PCP), the most prominent opportunistic infxn in AIDS patients
    • can be rapidly fatal if not controlled with medication


describe fungal culture medium

  • the medium commonly employed is Emmon's modification of Sabouraud Dextrose Agar (pH = 5.4)
  • the media may be supplemented with antibiotics such as gentamicin and chloramphenicol to minimize bacterial contamination and cyclohexamide to inhibit saprophytic fungi


describe hyphae 

  • hyphal structure reflects the phylogeny of the fungi
    • septate fungi taken from lesions are nearly always ascomycetes
    • aseptate fungi are zygomycetes
  • pseudohyphae are characteristic of Candida spp. infxns
  • low angle branching is characteristic of Aspergillus spp. and right angle branching of Zygomycetes
  • coloration may be described as dematiaceous (dark) or hyaline (transparent)


describe spores

  • spore forming structures are either enclosed (sporangia) or open (conidiophores)
  • phialides are flask shaped cells from which conidia bud
  • arthrospores are ceclls that become conidia as they break off from the end of a hypha, in contrast to blastospores, which are produced by budding, such as at the ends of phialides or simply from the sides of hyphae
  • chlamydospores are thick-walled spores capable of surviving adverse conditions


describe fungal stains

  • sputum, washing and aspirates may be prepared with 10% KOH
    • only the chitinous hyphae survive
  • calcofluor white and/or Gram stain are also used
    • calcofluor white binds chitin and fluoresces under UV illumination
    • Candida albicans stains Gram positive
  • Gomori methenamine silver staining is a popular histochemical stain used to demonstrate hyphae, spores and conidiophores 


describe fungal serology

  • detection of anti-fungal antibody is helpful in diagnosis of sub-cutaneous and systemic mycoses and for the prognosis and response to anti-fungal drugs


describe conidial structure

  • macroconidia and microconidia refer to the size of spores
    • Zygomycetes may form both types as blastospores
  • Tuberculate refers to the knobby apperance of a spore
    • Histoplasma capsulatum forms tuberculate macroconidia as characteristic spores


describe fungal cultures

  • fungi are usually easily cultured on general bacterial media or Sabouraud agar
    • Sabouraud agar is too acidic to support bacterial growth
  • dimorphic fungi are usually demonstrated by the growth of yeast colonies at elevated temperature
    • Candida albicans is the reverse however, and the growth of pseudohyphae is favored at elevated temperature


describe the control of fungal infections

  • control involves use of anti-fungal compounds that target specific metabolic pathways
    • inhibition of ergosterol, beta-glucan and chitin biosynthetic pathways, inhibition of DNA synthesis by depleting thymine pools